New Zealand's seas could cool or overheat under an El Niño on track to set a record

The analogue record has only three events in it ^[4][18]. They agree well enough about what happens on land. For the sea, the authors of an article published by phys.org write that the picture is less clear and that ecological effects will likely be mixed ^[6].

The usual expectation of cooler water comes from the wind change that drives El Niño. In a normal year, easterly trade winds pull cold, nutrient-rich water up off the coast of South America ^[15]. During El Niño those winds weaken or reverse, that upwelling stops, and warm surface water collects in the eastern Pacific ^[15]. Around New Zealand the typical result is slightly cooler-than-average sea surface temperatures. According to the authors, this dip could give heat-stressed ecosystems short-term relief ^[7].

The 2015-16 event complicates that. Its Tasman Sea heatwave lasted about eight months ^[11][19]. The authors say heatwaves can occur alongside El Niño, and that they are getting longer, more intense and more frequent with climate change ^[10]. The thing this record doesn't tell you is whether El Niño helped produce the 2015-16 heatwave or simply failed to prevent it. The article is direct on that point: the exact influence of El Niño on ocean currents and residual heating in the region is unknown, as are the potential impacts farther north ^[17]. One heatwave in three analogue years is a small sample, and the article does not put a probability on one forming this summer.

The heatwave case matters more than its frequency suggests because of the damage it does. If a marine heatwave occurs this summer, the authors write, it may have outsized impacts on the coast compared with those of El Niño itself ^[14]. In Australia, warm water has been tied to new disease outbreaks among farmed oysters and to wild pāua dying off ^[13]. This event also starts from a higher baseline. The article says temperature rises may be particularly dramatic because climate change has already pushed air and ocean temperatures above normal ^[3].

Even the cool scenario has a cost for a west-coast farm. The westerlies that may lift upwelling and feed kelp and phytoplankton ^[8] come with heavier rain. That rain can carry sediment from logged or farmed catchments onto filter-feeding shellfish ^[9].

"Forecasts are fickle, unexpected outcomes are likely and preparation is needed," the authors wrote ^[16]. I think that advice is right for this summer. It rests on three analogue years and an admitted gap in the regional physics, and a clearer account of how this El Niño moves heat through the Tasman is the evidence that would narrow it.